1. Field of the Invention
The present invention relates to an ultrasonic motor, and more particularly to its rotor structure.
2. Description of Related Art
An ultrasonic motor using standing wave has been known hitherto. An example of this kind of ultrasonic motor is shown in FIG. 10A. The motor is composed of stator 310 and rotor 330 which rotates relative to the stator. Stator 310 includes first block 312, first electrode plate 317, first piezoelectric element 315, second electrode plate 318, second piezoelectric element 316, and second block 314, all of these elements being assembled into a single body with through-bolt 319 and nut 320 as shown in FIG. 10A. Rotor 330 is rotatably assembled on the top of stator 310 with the same through-bolt 319 and nut 320. On second block 314, plural slanted slits 313a are formed.
High frequency voltage is imposed on both piezoelectric elements 315, 316. The voltage is imposed on first piezoelectric element 315 from terminals 317b, 318b connected to both electrode plates 317, 318, respectively. The voltage is imposed on second piezoelectric element 316 from terminal 318b and the lower surface of second piezoelectric element 316 which is electrically connected to terminal 317b through bolt 319. Both piezoelectric elements 315, 316 generate longitudinal vibration which is transmitted to both blocks 312, 314. The longitudinal vibration transmitted to second block 314 generates twisting vibration as a function of slanted slits 313a formed on upper part 313 of second block 314. The longitudinal and twisting vibrations are combined in stator 310 forming ellipse vibration on the upper surface of rotor 330 on which rotor 330 is rotatably mounted. Rotor 330 is rotated by the combined vibration of stator,310. Some more details of the combined vibration in stator 310 are schematically shown in FIG. 10B. The twisting vibration is shown by arrows in the left and the longitudinal vibration is shown in the right. The longitudinal vibration gives the rotor a lifting force, while the twisting vibration gives the rotor a rotating force.
There is a problem in the conventional ultrasonic motor in that obtaining a sufficient rotational speed of the rotor is difficult since the twisting vibration converted from the longitudinal vibration is not sufficiently large. Moreover, the twisting vibration tends to vary according to conditions of fastening the elements of the stator including the fastening strength.